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1. The emission of biogenic gases, particularly methane, is usually associated with wetlands rather than clean streams. Here, we investigated methane production from a southern English chalk stream, where increased sedimentation, compounded by extensive macrophyte growth, may have altered ecosystem function. 2. Cover of the channel by the dominant macrophyte, Ranunculus penicillatus, peaked in August, when plant beds were associated with low water velocity and the accumulation of sediment (<2000 lm) dominated by the sand-sized fraction (63-1000 lm). 3. Over spring and summer there was a marked increase in the silt/clay fraction of the sediment, a concomitant drop in mean particle size and, hence, inferred permeability. At the same time there was an increase in CH 4 transport through Ranunculus stems and an increase in water column CH 4 concentration, while the sediment CH 4 concentration increased 100-fold between February and April. A marked seasonal enrichment in the d 15 N of N 2 dissolved in the pore water correlated with CH 4 flux and, coupled to the shift in particle size, suggested a transient input of organic matter, possibly of terrestrial origin. 4. Potential areal methane production and measured efflux were similar to that from some U.K. peatlands and represent one of the first accounts of significant methanogenesis to be measured in a stream channel. Most (>90%) of the methane flux is transported to the atmosphere through the Ranunculus stems. 5. Although the total flux of methane from U.K. chalk streams is probably relatively modest (estimated at 3.2 · 10 )6 Tg CH 4 year )1 ), this phenomenon changes our perception of the health of these ecosystems and indicates another deleterious side effect of agriculture.

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Abstract:Excessive fine sediment deposition on the river channel bed together with colmation of finer sediments within the hyporheic are now linked to the degradation of the aquatic habitats of gravel bed rivers in permeable catchments. Previous studies of chalk rivers (associated with outcrops of calcareous rock) have demonstrated the important role of aquatic vegetation in trapping fine sediment on the river channel bed. This research investigated the spatio-temporal patterns and composition of fine sediment stored in two vegetated river reaches, in the Frome and Piddle catchments, Dorset (UK), with contrasting hydrological regimes, in order to establish the importance of aquatic vegetation in controlling the magnitude and timing of sediment storage in chalk rivers.Monthly mapping of macrophyte and sediment cover at the two sites (Maiden Newton and Snatford Bridge, 2003-2004) revealed a cyclical pattern of sediment storage related to the growth and die-back of aquatic vegetation peaking at 66Ð8 kg m 2 in July 2003 at Maiden Newton, and 23Ð5 kg m 2 in October 2003 at Snatford Bridge. Sediment was stored within gravels and beneath vegetation in the margins and mid-channel locations at both sites. Significantly more sediment was stored beneath vegetation than within gravels. The spatio-temporal pattern of sediment storage at the reach scale and the composition of the stored sediments reflected the growth patterns and functional form (flexibility) of the dominant macrophytes Ranunculus penicillatus subsp. pseudofluitans (water crowfoot) and Rorippa nasturtium aquaticum (watercress). Finally, the paper discusses the implications of reach-scale patterns in sediment storage for contaminant storage.